40FF Bra Size (UK)
Everything about the 40FF bra size in United Kingdom sizing: what it means, its measurements, whether it runs big or small, sister sizes, and conversions to US, EU, FR & AU.
What is a 40FF bra size?
A 40FF in United Kingdom sizing is a bra with a 40 band and a FF cup. “40” is the band in inches. UK and US bands are identical. “FF” is the cup, from a 8-inch difference between the bust and the band (one inch per cup letter). The same body is written as US 40H, EU 90H, FR 105H, AU 18FF, IT 6H.
Someone who measures roughly 39 inches (99 cm) around the ribcage and about 48 inches (122 cm) around the fullest part of the bust lands at 40FF using the modern snug-measure method. The band carries most of the support; the cup describes the breast volume the bra needs to hold: measured relative to that band.
“40” is the band in inches. UK and US bands are identical. It sets how the bra fits around the ribcage and provides about 80% of the support.
FF is a full-bust cup: a 8-inch bust-minus-band difference. It reflects breast volume, always relative to the 40 band.
Band and cup are one size. A 40FF is not the same volume as a 36FF: the cup letter only has meaning next to its band.
40FF measurements
A 40FF comes from two measurements: a snug underbust that gives the 40 band, and a looser bust. The cup is the gap between the two: for FF, that gap is 8 inches.
| Measurement | Range | Sets |
|---|---|---|
| Underbust (snug) | 38.5–40.4″ · 98–103 cm | Band 40 |
| Bust (fullest) | 47.5–48.4″ · 121–123 cm | — |
| Difference | 8″ · ≈ 20 cm | Cup FF |
Underbust rounds to the nearest even band; an odd reading (e.g. 39″) can go either way, so try 38G and 40FF and let the band decide.
Measurements vary by body shape, so treat these as the center of a range. UK band labels are converted from the US band by convention rather than read directly in centimeters, which is why a converted size can feel a step different from a bra drafted natively in this system.
Is 40FF big or small?
40FF is a full-bust cup on a fuller band: carried by full-bust specialists more than mainstream ranges. But the honest answer is that a cup letter is not a fixed amount of volume: it grows with the band. Tap a size below to see how.
All five are FF cups, yet the volume grows with the band. 40FF is the highlighted size.
Because the cup is measured relative to the band, a 40FF holds more than a 36FF and less than a 44FF, even though all three are “FF cups.” So “big” or “small” only has meaning once you know the band.
If 40FF cups fit well but the band feels off, you don't need a different cup: you need a sister size. And if you're unsure 40FF is your starting size, the calculator settles it from two measurements.
Were you measured as 40FF correctly?
Most people wearing the wrong size were sized by the old “+4 rule”: adding four inches to the underbust to get the band. It inflates the band and, because the cup is the difference from that inflated band, understates the cup by about two letters.
Same body, same volume: the old rule pushes it into a looser band and a smaller cup.
The tell-tale signs you were sized with the old method: the 40 band rides up at the back, you fasten it on the tightest hook from day one, and the straps do most of the work. If that sounds familiar, 36GG is the size to try first: it holds the same volume on a band that actually supports.
Key takeaway: if a calculator adds inches to your underbust, its 40FF is not the same 40FF as ours. Brasize.ai uses the modern snug-measure standard used by professional fitters.
Which bra styles fit a 40FF best?
At this depth the bra is doing real load-bearing work. Prioritise full-cup and side-support constructions from full-bust specialists, firm (often three- or four-hook) bands, and wider straps. Molded single-piece cups rarely fit well here; seamed cups distribute weight and shape better. On a 40 band, prioritise firm, wide band elastic and a taller back; a soft, narrow band will roll and ride up regardless of the label. Longline and three-hook styles help.
Full-cup and side-support constructions from full-bust specialists.
Look for a three-hook band with firm, wide elastic: the band does the lifting.
Wider, cushioned straps and a wire that fully encloses the breast root.
Sister sizes share cup volume but not cup shape: the wire on a smaller band is narrower and more projected, the wire on a larger band is wider and shallower. If you have a narrow breast root, the snugger sister will often fit better; if your root is wide, the looser sister may: even when the band tension of the original was right.
40FF sister sizes
Sister sizes hold the same cup volume as 40FF on a different band. They're the fix when the cups fit but the band is too loose or too tight.
| Size | Band feel vs 40FF | Cup volume | When to try it |
|---|---|---|---|
| 36GG | Two bands snugger | Same as 40FF | Only if 38G is still loose; wire will be noticeably narrower |
| 38G | One band snugger | Same as 40FF | 40 band rides up or feels loose |
| 40FF | Your starting size | — | Band sits firm and level |
| 42F | One band looser | Same as 40FF | 40 band digs in or feels tight |
| 44E | Two bands looser | Same as 40FF | Rarely right: the band gives up too much support |
The rule: go down a band and up a cup (40FF → 38G) for the same cups on a tighter band, or up a band and down a cup (40FF → 42F) for the same cups on a looser band. Sister sizes share cup volume but not cup shape: the wire on a smaller band is narrower and more projected, the wire on a larger band is wider and shallower. If you have a narrow breast root, the snugger sister will often fit better; if your root is wide, the looser sister may: even when the band tension of the original was right.
Signs 40FF is too small, or too big
Fix: go up a cup on the same band → 40G. If the band is also loose, go to 38G.
Fix: gaping cups → down a cup, 40F. Band riding up with cups that fit → 38G.
40FF in US, EU, FR & AU sizing
The same 40FF body measurements are written differently around the world. Bands and cup letters diverge by region, so convert deliberately when buying from an international brand.
| Region | Size | How the band is derived |
|---|---|---|
| United States (US) | 40H | Band in inches |
| United Kingdom (UK) | 40FF | Same band as US; cups diverge above DD |
| Europe (EU) | 90H | US band × 2.5 − 10 = 90 |
| France / Spain (FR) | 105H | EU band + 15 = 105 |
| Australia / NZ (AU) | 18FF | US band − 22 = 18 |
| Italy (IT) | 6H | (EU − 60) ÷ 5 = 6 |
| Japan (JP) | 90H | EU band; cups close to EU |
| Germany (DE) | 90H | EU sizing |
Cup letters agree across systems up to D. Above D they diverge (a US DDD is a UK E and an EU F) which is where the size converter earns its keep. Brands that size natively in UK: Panache, Freya, Curvy Kate, Bravissimo.
How to tell if 40FF is your correct size
A calculated 40FF is an accurate starting point. Confirm it on the body: band, cups, gore, straps.
- Band sits firm and level. The 40 band should be snug around your ribcage with room for two fingers, not riding up at the back. It gives most of the support.
- Cups fully contain the breast. No spillage over the top or sides, and no gaping or wrinkling in the FF cups. Gaping usually means down a cup; spillage means up a cup.
- Center gore lies flat. The panel between the cups should sit against your sternum, not float away.
- Straps stay put. If you're tightening straps to feel supported, the 40 band is probably too big: try 38G.
If the cups fit but the band is wrong, move to a sister size rather than changing cup letters. There's no universal standard between brands: a 40FF in one label can fit like the size on either side in another, so use the fit checks, not just the label. Full symptom-by-symptom guide: bra fit guide.
Educational resource, not medical advice. Read our editorial policy.
HauteFlair labels cups in UK letters, so these links already convert your size by position (a US DDD lands on a UK E, a US G on a UK F). HauteFlair is owned by the same company as Brasize.ai; that relationship never changes the sizing math, charts, or fit advice on this page. How this site makes money.
Is 40FF actually your size?
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Calculate My Bra Size →40FF bra size: answered
The questions people ask most about the 40FF size: its volume, sister sizes, conversions and fit.
Is 40FF a big bra size?
40FF is a full-bust cup on a 40 band. A cup letter is a ratio, not a fixed volume: FF means a 8-inch difference between bust and band, so a 40FF holds more than a 36FF and less than a 44FF. “Big” only has meaning once you know the band.
What is the sister size of 40FF?
The sister sizes of 40FF are 38G (snugger band) and 42F (looser band). Sister sizes hold the same cup volume as 40FF on a different band. Move to one when the FF cups fit but the 40 band is too loose or too tight.
What is 40FF in European sizing?
A UK 40FF is an EU 90H. EU bands follow US × 2.5 − 10; cup letters match up to D, then EU continues the alphabet (E, F, G) where the US repeats D.
What is 40FF in US sizing?
A UK 40FF is a US 40H. UK and US bands are identical; UK E = US DDD, UK F = US G, UK FF = US H.
What measurements are a 40FF?
Roughly a 39-inch (99 cm) snug underbust and a 48-inch (122 cm) bust at the fullest point. The 8-inch difference gives the FF cup; the underbust gives the 40 band. Treat these as the center of a range.
What is 40FF in centimeters?
A 40FF corresponds to an underbust of about 98–103 cm and a bust of about 121–123 cm: a difference of roughly 20 cm. In metric-labelled systems the same size is EU 90H, FR 105H and JP 90H.
Which bra styles suit a 40FF?
At this depth the bra is doing real load-bearing work. Prioritise full-cup and side-support constructions from full-bust specialists, firm (often three- or four-hook) bands, and wider straps. Molded single-piece cups rarely fit well here; seamed cups distribute weight and shape better. On a 40 band, prioritise firm, wide band elastic and a taller back; a soft, narrow band will roll and ride up regardless of the label. Longline and three-hook styles help.
How do I know if 40FF is my correct size?
Check four things: the 40 band sits level and snug on the loosest hook, the FF cups are filled without spillage or gaping, the center gore lies flat against the sternum, and the straps stay put without carrying the weight. If the band is wrong but the cups fit, try a sister size such as 38G.
Was I sized as 40FF using the old +4 method?
If your 40 band rides up, fastens on the tightest hook from new, and the straps carry the weight, probably yes. The +4 rule adds four inches to the underbust; the same body measured snugly is closer to 36GG, same cup volume, a band that supports.